1. Field of the Invention
The present invention relates to a long-optical-element holding device, a method of adjusting shape of a long optical element, a long-optical-element shape adjusting device, a method of correcting scanning line variation, an optical scanning device, and an image forming apparatus.
2. Description of the Related Art
In recent years, in image forming apparatuses, such as a laser printer, a digital copying machine, and a laser facsimile, a plastic material is often used as a material for an optical element forming a scanning optical system. Although plastic is excellent in mass productivity, a shape of the plastic often varies from an ideal one because of, for example, a distribution of temperature in a die at the time of molding and non-uniformity of cooling after the plastic is taken out from the die.
In the optical scanning system, an optical element elongated in a main scanning direction is often used. Since the optical element bends in a sub-scanning direction, depending on a holding method for the long optical element, the bending of the optical element causes scanning position deviation in a direction corresponding to sub-scanning, such as scanning line tilt and scanning line bending. An error in attachment of the optical element to a housing also causes scanning position deviation in a direction corresponding to sub-scanning on a surface to be scanned. The scanning position deviation is often nonnegligibly large.
In an image forming apparatus having a plurality of scanning units for an increase in speed of image formation or for color image formation (hereinafter, “tandem image forming apparatus”), an amount of scanning position deviation in a direction corresponding to sub-scanning such as scanning line bending is different for each of the scanning units because of a temperature deviation among housings that hold and fix the scanning units. This causes deterioration in a quality of a formed image, color misalignment, and the like. The same problem occurs in a system for making a plurality of light beams incident on a single deflector to use the light beams for scanning and arranging optical elements to be laid one on top of another in a sub-scanning direction, that is, a system for holding all scanning units in an identical optical housing. An amount of scanning position deviation in a direction corresponding to sub-scanning such as scanning line tilt and scanning line bending on each surface to be scanned, for example, a photosensitive member surface is different because of an influence of a shape error or an attachment error of the optical elements forming scanning optical systems or a temperature distribution in an identical housing. This causes deterioration in a quality of a formed image.
A tandem image forming apparatus is one of the types of a full-color image forming apparatus. In the tandem image forming apparatus, for example, four photosensitive drums are provided along a conveyance surface of a transfer belt in association with colors of cyan (C), magenta (M), yellow (Y), and black (K). Optical scanning devices provided in association with the respective photosensitive drums scan the photosensitive drums using light beams to form electrostatic latent images on peripheral surfaces of the photosensitive drums. The electrostatic latent images are visualized with toners of corresponding colors. The visualized images are sequentially transferred onto a sheet conveyed by the transfer belt to be superimposed one on top of another to form a multi-color image. Therefore, if scanning position deviation in a direction corresponding to sub-scanning occurs for each color, deterioration in a quality of a formed image or color misalignment is caused.
As a solution for the problem described above, many proposals have been made. Some of proposals related to the present invention among the proposals are described below as examples.
As an example, an optical scanning device is proposed in, for example, Japanese Patent Application Laid-Open No. H11-231240. The optical scanning device includes an adjusting mechanism for forcibly deforming an optical member, for example, a mirror in a scanning optical system with an adjusting screw or the like to correct scanning line bending. However, this system has a difficulty in that a magnification in a main scanning direction changes because of the correction of scanning line bending. Therefore, in particular, in a tandem drawing optical system, dot position deviation in the main scanning direction occurs among optical scanning devices and stations consisting of photosensitive members corresponding to the optical scanning devices. Color misalignment due to the dot position deviation occurs. Since it is impossible to adjust scanning line tilt, even if an amount of scanning line bending for each of the stations is reduced to be within a predetermined range, if the scanning line tilt fluctuates for each of the stations, color unevenness or color misalignment is caused. This leads to deterioration in a quality of a formed image.
As another example, a scanning optical device is proposed in, for example, Japanese Patent Application Laid-Open No. 2001-166235. The scanning optical device presses and supports a plastic lens disposed in a housing with a plurality of adjusting screws screwed in the housing and springs arranged in positions opposed to the adjusting screws and adjusts a pressing force by the adjusting screws to correct deflective deformation of the plastic lens and correct any one of bending and tilt or both of a scanning line on a surface to be scanned. Since the plastic lens is disposed in the housing, the adjustment system according to the invention has a difficulty in that a size of the optical scanning device increases. Since the housing is fixed to an optical housing, it is impossible to correct the scanning line tilt.
As still another example, a tandem image forming apparatus using a plurality of scanning units is proposed in, for example, Japanese Patent Application Laid-Open No. 2001-166235. The tandem image forming apparatus is a color image forming apparatus that uses a scanning optical device in which all the scanning units are assembled in separate housings, respectively, and positions of all the housings are adjusted to photosensitive members corresponding thereto to cause scanning lines in the photosensitive members to coincide with one another. However, according to the invention, there is a difficulty in that, since a mechanism for adjustment is complicated and adjustment time is long, an increase in cost is caused. It is likely that color misalignment occurs because it is impossible to adjust scanning line bending when temperature fluctuates from a room temperature state to cause a temperature distribution among the scanning units. It is impossible to cope with aged deterioration due to a temperature change or the like. Thus, it is impossible to highly accurately correct color misalignment during printing or in an environment of use.
As still another example, an optical scanning device for a multi-color image forming apparatus is proposed in for example, Japanese Patent Application Laid-Open No. H10-268217. The optical scanning device brings an adjusting screw into abutment against a portion near the center in a length direction of a toroidal lens consisting of a plastic lens forming a scanning optical system to bend the toroidal lens in a sub-scanning direction and adjusts a deflection degree of the toroidal lens according to adjustment of the adjusting screw to correct scanning line bending. According to the invention, it is possible to correct scanning line bending among photosensitive members in an initial state. However, it is impossible to cope with aged deterioration due to a temperate change or the like. Thus, it is impossible to highly accurately correct color misalignment due to a change in environment during printing or color misalignment due to an environment of use.
As described above, all the conventional technologies still have a technical problem in that a dot position in a main scanning direction in a surface to be scanned is shifted, it is impossible to correct scanning line tilt, or it is impossible to correct scanning line bending involved in any one of aged deterioration and a temperature change or both.
Further, conventionally, as a constitution of an optical scanning device, it is proposed to correct scanning line bending and scanning line tilt by bending a long lens forming a scanning and focusing optical system with a plurality of fulcrums as supporting points or tilting the long lens in a sub-scanning direction in order to reduce the bending or the scanning line tilt (see, for example, Japanese Patent Application Laid-Open No. 2002-258189).
In an image forming apparatus using an electrophotographic process called a Carlson process, formation of a latent image by exposure, a development by a toner, and transfer of a toner image to a transfer member are performed according to rotation of a photosensitive drum serving as an image carrier. Therefore, in a multi-color image forming apparatus in which a plurality of photosensitive drums are arranged along a conveyance direction of a transfer member to superimpose toner images formed by image forming stations of respective colors one on top of another, a time difference from formation to transfer of latent images due to eccentricity and fluctuation in diameters of the photosensitive drums, a difference of intervals of the photosensitive drums of the respective colors, or fluctuation in speed or meander of a transfer member such as a transfer belt or a conveyor belt for conveying recording paper causes misregistration in a sub-scanning direction of respective toner images. This leads to color misalignment and color change to deteriorate an image quality.
Conventionally, regardless of whether the misregistration is misregistration caused by an optical scanning device or misregistration due to other reasons, the misregistration is periodically detected according to a misregistration detection pattern recorded in a transfer member at the time of start of the apparatus, between jobs, or the like. Concerning a sub-scanning direction, a position of a leading line is corrected by adjusting timing for starting drawing an image every other deflective reflection surface of a polygon mirror. Concerning a main scanning direction, a position for starting drawing an image is corrected by adjusting light-emission start timing of a light source according to a synchronization detection signal (see, for example, Japanese Patent Application Laid-Open No. H7-19084, Japanese Patent Application Laid-Open No. 2001-253113, and Japanese Patent Application Laid-Open No. 2003-154703 (hereinafter, “first document”)). Along with the correction, full-width magnifications of respective colors are set by taking measures for, for example, detecting scanning time from a scanning start to a scanning end and adjusting a frequency of a pixel clock to the scanning time detected (see, for example, Japanese Patent Application Laid-Open No. H9-58053).
On the other hand, in such a multi-color image forming apparatus, an increase in speed and an increase in density have been advanced year after year. As measures for coping with the increase in speed and the increase in density, there is a method of increasing the number of revolutions of a polygon motor. However, since there is a limit in a life of a bearing, heat generation and vibration are not sufficiently controlled. Thus, a system using a multi-beam light source that can realize the increase in speed and the increase in density by performing scanning using a plurality of beams at a smaller number of revolutions is proposed. However, the multi-beam light source has differences in pitches and wavelengths among light sources. Thus, as disclosed in the first document, an example for preventing the differences among the light sources by performing misregistration detection for a set of a plurality of lines separately is proposed. Moreover, an example using a liquid crystal deflection element as means for correcting a scanning position in a sub-scanning direction is proposed (see, for example, Japanese Patent Application Laid-Open No. 2003-233094 and Japanese Patent Application Laid-Open No. 2003-215484).
In recent years, intending to improve a scanning characteristic, an optical element having a special surface represented by an aspherical surface is generally adopted for a focusing optical system of an optical scanning device. There is a resin material as a material from which such an optical element having a special surface can be easily formed. Since it is possible to form the special surface by integrally molding the resin material, cost for forming the optical element is low. Thus, the optical element formed of the resin material is frequency used in a focusing optical system.
An optical characteristic of the optical element formed of the resin material is affected by a change in temperature or humidity to change easily. When such an optical element is used for a scanning and focusing optical system for an optical scanning device, a state of bending and constant speed property of a scanning line also change because of a change in temperature or humidity. Therefore, when several tens color images are continuously formed, an internal temperature rises because of a continuous operation of respective optical scanning devices, an optical characteristic of the focusing optical system changes, and a state of bending and constant speed property of scanning lines for drawing images in the respective optical scanning devices gradually change. As a result, a tone of a color image obtained in an initial period and a tone of a color image obtained in an end period may be completely different because of a color misalignment phenomenon. Since temperature in the optical scanning device rises, a housing for the optical scanning device expands and a position of a receiving surface of the optical element changes. Consequently, a deficiency occurs in that a scanning position with respect to a photosensitive member is shifted over time, for example, a beam position with respect to the optical element changes or a setting angle for a return mirror changes.
A scanning and focusing lens such as an fθ lens representative as an optical element forming a scanning and focusing optical system is generally formed as a strip-shaped lens long in a main scanning direction by cutting off an unnecessary portion (a portion on which deflected light beams are not made incident) of a lens in a sub-scanning direction. When the scanning and focusing optical system is formed of a plurality of lenses, a lens length in the main scanning direction increases as an arrangement position of the lenses is farther apart from a light deflecting unit. A long lens having a length of about 100 mm to 200 mm or more is required. Such a long lens is generally formed by resin molding using a resin material. When a temperature distribution in the lens becomes non-uniform because of a change in ambient temperature, the lens is warped to be bowed in the sub-scanning direction. Such warp of the long lens causes the scanning line bending. When the warp is marked, the scanning line bending also occurs excessively.
As examples of the optical scanning device applicable to the multi-color image forming apparatus, a constitution in which light beams from light sources corresponding to respective colors are collectively used for scanning by a single polygon mirror and a plurality of mirrors for leading the respective light beams to scanning optical systems and photosensitive drums corresponding to the light beams are integrally supported in a common housing and a constitution in which optical scanning devices are arranged individually in association with respective photosensitive drums are known (see, for example, Japanese Patent Application Laid-Open No. 2002-148551 (hereinafter, “second document”)). According to the constitution described in the second document, components are arranged such that light beams traveling to the photosensitive drums pass different paths. Thus, respective irradiation positions easily fluctuate because of, for example, temperature of an environment in which the multi-color image forming apparatus is set.
It is possible to periodically detect a misregistration detection pattern recorded in a transfer member such as an intermediate transfer belt, for example, at the time of start of the apparatus or between jobs and correct the deviation of an irradiation position based on a result of the detection. However, since the irradiation position further fluctuates because of heat or the like from a fixing device and a polygon motor following a print operation, if the number of prints per one job is large, color misalignment and color change gradually occur. In particular, as in the invention described in the second document, when optical systems are arranged to be opposed to each other across a polygon mirror, since scanning directions are opposite in the opposed optical systems, a position for drawing an image is shifted by fluctuation in a main scanning magnification. Moreover, since scanning position deviation among respective colors increases because of distortion of a housing, color misalignment and color change tend to occur.
As measures against the problem, it is conceivable to always observe temperature and, when the temperature reaches a predetermined temperature change width or when the number of prints exceeds a predetermined number, stop the print operation and correct deviation of the irradiation position again. However, taking into account a flow of creating a misregistration detection pattern, detecting and correcting the misregistration detection pattern, creating a detection pattern again, and detecting and correcting the detection pattern, it takes several minutes until the correction ends. This deteriorates productivity and a toner is consumed wastefully to form the detection pattern. Therefore, it is desirable to minimize frequency of correction of a light beam irradiation position.
In the image forming apparatus using the Carlson process, formation, development, and transfer of a latent image are performed according to rotation of a photosensitive drum. Therefore, in a tandem multi-color image forming apparatus in which a plurality of photosensitive drums are arranged along a conveyance direction of a transfer member to superimpose toner images formed by image forming stations of respective colors one on top of another, a time difference from formation to transfer of latent images due to eccentricity and fluctuation in diameters of the photosensitive drums, a difference of intervals of the photosensitive drums of the respective colors, or fluctuation in speed or meander of a transfer member such as a transfer belt or a conveyor belt for conveying recording paper causes misregistration or skew of respective toner images. This leads to color misalignment and color change to deteriorate an image quality.
Similarly, in the optical scanning device, unless changes (indicating bending, tilt, curve, deformation, and the like) of scanning lines for electrostatic latent images formed on a photosensitive drum are accurately adjusted, bending or skew of scanning lines for forming respective toner images occurs. This also causes color misalignment and color change.
Conventionally, regardless of whether the misregistration or the skew is misregistration or skew caused by an optical scanning device or misregistration or skew due to other reasons, as disclosed in Japanese Patent Application Laid-Open No. H7-199084 and Japanese Patent No. 3049606, the misregistration or the skew is periodically detected according to a detection pattern recorded in a transfer member at the time of start of the apparatus, between jobs, or the like. The misregistration is corrected by changing a position of a leading line by adjusting timing for starting drawing an image every other surface of a polygon mirror.
The skew is corrected by changing scanning line tilt by, for example, tilting a return mirror as disclosed in Japanese Patent No. 3049606 or rotating a scanning lens having a convergent action in a sub-scanning direction around an optical axis as disclosed in Japanese Patent Application Laid-Open No. H11-153765 and Japanese Patent Application Laid-Open No. 2003-262816.
In this way, the misregistration or the skew is automatically corrected by providing a mechanical correction function and building a pulse motor or the like in the optical scanning device.
On the other hand, the scanning line bending is also mechanically corrected by, for example, curving a return mirror as disclosed in Japanese Patent No. 3049606, correcting a shape along a main scanning direction of a scanning lens in a sub-scanning direction as disclosed in Japanese Patent Application Laid-Open No. 2002-148551, or rotating a position of a scanning lens around an axis orthogonal to a sub-scanning section as disclosed in Japanese Patent Application Laid-Open No. 2003-255245.
As a constitution for controlling deviation of a plurality of scanning lines, there is one disclosed in Japanese Patent Application Laid-Open No. 2003-140070. As a technology related to the invention, there is one disclosed in Japanese Patent Application Laid-Open No. H10-228148.
However, recently, since a scanning lens is made of resin according to a reduction in cost, it is difficult to secure straightness of a focal line of the scanning lens because of warp at the time of molding, distortion of a lens surface, and a distribution of internal refractive indexes.
On the other hand, since users have strict views about color misalignment and color change, higher accuracy is required for correction of the misregistration, the skew, and the scanning line bending.
Since the detection by the detection pattern requires labor and time for the scanning line bending, it is difficult to correct fluctuation of the bending due to fluctuation in temperature or the like. The point is how accurately the bending is adjusted at the time of manufacturing.
In that regard, in the conventional method of correcting scanning line bending, warp of a return mirror or a position of a scanning lens is changed. Although it is possible to correct the bending, a scanning line on a surface to be scanned is simply curved in a shape of a quadratic function curve to offset the curve. Thus, the method cannot cope with complicated bending having a quadratic or higher-order function curve component.
For example, a method of correcting bending and tilt while correcting warp of an external shape of a scanning lens by holding the scanning lens between metal plates is proposed in Japanese Patent Application Laid-Open No. 2004-109761. However, even if the external shape is straight, if there is distortion of a lens surface or a distribution of internal refractive indexes, it is impossible to cope with a complicated bent shape as in the conventional correction method.
In general, a quadratic function curve component is caused by warp of a scanning lens, eccentricity of an optical axis with respect to an attachment reference surface, distortion of a transfer member, or the like. On the other hand, a higher-order function curve component is caused by combination of bending of focal lines in a plurality of lenses. Thus, the higher-order function curve component has less importance compared with the quadratic function curve component.
Therefore, the higher-order function curve component has been treated as a correction residual. However, the higher-order function curve component is innegligible because of complication of a bent shape and a reduction in an allowable residual due to the distortion of a lens surface involved in formation of a scanning lens with resin, the distribution of an internal refractive indexes, and the like.